This invention relates to methods for determining the thickness of both opaque or transparent samples (e.g., thin films).
Thin films of dielectric (e.g., polymer, oxide) and conducting (e.g., metal) materials are used in a range of microelectronic, optical, and biomedical devices. A microprocessor, for example, contains multiple layers of metal, semiconducting, and oxide thin films. Thickness variations in these films can modify the films' electrical and mechanical properties, thereby affecting the performance of the microprocessor. Accordingly, film thickness is often monitored as a quality-control parameter during and/or after the microprocessor's fabrication.
Well-known techniques, such as optical ellipsometry and interferometry, measure the thicknesses of transparent films. Measuring the thicknesses of opaque films (e.g., metal films) is a more difficult problem. Current methods for measuring metal film thickness include electrical tests and x-ray spectroscopy. In the electrical test (commonly called a sheet-resistance test), a pair of conducting probes contact the film; electrical resistance, as measured by the probes, is proportional to the film's thickness. Sheet-resistance tests require contacting the film, making such tests undesirable for measuring actual devices during the fabrication process. X-ray-based techniques measure the thickness of metal films by inducing, measuring, and analyzing x-ray fluorescence. This method is both non- contact and non-destructive, but requires large, unwieldy instruments that generate hazardous x-ray radiation.